It is known that this type of grinder makes use of centrifugal force to project materials which are to be ground onto targets at very high speeds, the whole arrangement being disposed in a vacuum in order to avoid the braking of the particles through air resistance.
A vacuum grinder comprises a closed pressureresistant vessel which is under vacuum and at the top of which a distributor wheel rotated at high speed is disposed. The wheel is provided in its axis with a central feed chamber provided at the top with an axial aperture disposed at the bottom of a hopper fed with material to be ground by means of a metering device, for example of the screw type, disposed at the outlet of a feed chamber forming an air lock and enabling the material to be introduced into the vacuum vessel.
The distributor wheel is also provided with a plurality of projection channels whose axes are centered in a median plane at right angles to the axis and lead in the inward direction into the feed chamber and in the outward direction onto the periphery of the wheel.
The material introduced by the metering device into the central feed chamber is thus entrained by centrifugal force into the channels and projected, at the outlet of the latter, onto an assembly of plates forming targets and disposed all around the wheel, along the side wall of the vessel. The bottom part of the latter is in the form of a hopper and collects the fine powder formed by the bursting of the grains of material thus thrown onto the targets through the channels in the wheel.
If the distributor wheel rotates at a sufficiently high speed, a radial, tangential acceleration is thus created inside the channels and enables the desired speed to be obtained at the outlet. Inside these channels there is produced an effect of contact between the particles and the wheel in dependence on the speed of rotation, so that considerable wear ensues. This abrasive action depends on the physical properties of the particles, but is always very considerable as soon as the ejection speed itself becomes substantial, in view of the high value of the effect of contact between the particles and the wheel and the relative speed of displacement of the particles in the channels.
In order to protect these channels, French patent application No. 85-02234 in the name of the applicant discloses a distributor wheel in which the particle guide face in each channel has a positive curve, i.e. a curve winding in the direction of rotation of the distributor wheel and having a contour, judiciously calculated in accordance with the coefficients of friction of the materials in contact, which effects the attachment to this curve of a stable selfprotection layer consisting of the particles themselves, with automatic regeneration of this layer simultaneously with the wear to which it is subjected.
Because of the formation of the channels, in this arrangement the support faces of the particles formed by the bottom of the channels are therefore protected by a layer of particles, but the end point of the bottom of these channels, which is situated at the ejection aperture, is a vulnerable point which should also be protected.
Furthermore, the displacement of the particles to be ground from the center to the periphery of the distributor wheel takes place at a certain speed, which cannot be eliminated without bringing the output of the apparatus to zero.
This end point can be protected by forming a slope with the aid of appropriate shapes of the distributor wheel. It is now known that at its intersection with the laying plane a conical slope has zero thickness, and this also constitutes a vulnerable point where wear may occur. This solution consequently cannot be adopted.
In order to eliminate wear on the distributor wheel and to achieve effective protection at the ejection aperture of the distribution channels, it is therefore necessary either to ensure that the particles move over slopes whose thickness is not zero, or to ensure that the contact force between the particles and the slope is zero, particularly when the thickness of the slopes is almost zero.